1. Field of the Invention
The invention is directed to a method for operating a hemotherapeutic device for determining hemodynamic parameters during an extracorporeal hemotherapy, as well as to a device for determining hemodynamic parameters during an extracorporeal hemotherapy.
2. Description of Related Art
In methods used in chronic blood purification therapy, such as hemodialysis, hemofiltration, and hemodiafiltration, blood is circulated through an extracorporeal circuit. Often an arteriovenous fistula is surgically inserted as an access route to the vascular system. It is likewise possible for an implant to be used. The term "fistula" referred to in the following is understood to be any type of connection between a vein and an artery of a patient.
The purpose of the vascular access route is to supply a blood flow at least as great as the extracorporeal blood flow specified by the pump in the extracorporeal hemodialysis circuit. Should such a blood flow not be supplied, due, for example, to vessel constrictions (stenoses), the arterial needle may possibly attach itself by suction to the vascular wall, thereby interrupting the extracorporeal circuit. In most cases, however, a portion of the extradorporeal blood flow, namely the difference between the extracorporeal blood flow and the blood flow streaming into the fistula, is recirculated in the extracorporeal circuit. This phenomenon is described as recirculation.
A consequence of the fistula recirculation is a reduction in the quantity of substances requiring dialysis that is removed from the body per unit of time. The recirculating portion does not pass through the body's capillary system and, therefore, is not invaded again by toxic substances. Therefore, this portion has a lesser role in the blood purification. If the effectiveness of the dialysis is substantially reduced because of failure to detect and compensate for the recirculation, then the morbidity rate of such patients will rise over the long term. Thus, an important method for assuring quality in a dialysis treatment is to measure the quality of the vascular access route.
Various methods are known for measuring fistula recirculation. Common to all of them is the measurement of a physical-chemical blood property, which must be variable in the venous blood. The physical-chemical property can be altered through direct action of the user or indirectly via the dialysate preparation unit. The occurrence of fistula recirculation can subsequently be determined or quantified, in that a change in this property is also verified in the arterial blood.
Such a method is described by U.S. Pat. No. 5,312,550 and EP 0 590 810 A1, where an indicator solution is injected into the venous line and monitored for its concentration in the arterial blood. It is also possible to bypass the step of injecting an indicator solution, because when fistula recirculation is at hand, a short-term temperature drop is produced in the dialyzing fluid circuit, and then spreads to the venous branch of the extracorporeal circuit, to then lead to a detectable rise in temperature in the arterial branch of the extracorporeal circuit (M. Kramer and H. D. Polaschegg, EDTNA-ERCA J. 19, 6 (1993)).
The measured recirculation cannot always be readily interpreted, since this technical parameter is not easily correlated with the actual physiological parameter of interest, namely the blood flow to fistula Q.sub.F. Thus, for example, in spite of constant fistula flow Q.sub.F, the recirculation varies with the blood flow Q.sub.B. Moreover, many measuring processes not only record the fistula recirculation, but also the sum of the fistula and cardiopulmonary recirculation (M. Kramer and H. D. Polaschegg, EDTNA-ERCA J. 19, 6 (1993)). The cardiopulmonary recirculation, which is defined as the fistula flow's fractional share in the cardiac output (l/min), refers to already dialyzed blood, which arrives via the cardiopulmonary system directly in the arterial branch of the extracorporeal circuit without passing through the capillary system. The change in blood temperature that occurs in the arterial blood pathway before the onset of the fistula recirculation is attributable to the cardiopulmonary recirculation. In addition, because the fistula recirculation and the cardiopulmonary recirculation overlap, it is more difficult to interpret the measuring results.
Therefore, it would be beneficial to directly measure fistula flow Q.sub.F. The Journal of Medical Engineering and Technology 8, 118 (1984), a publication of Aldridge et al., describes the first attempts made to perform such measurements. Individual recirculation measurements are taken using the thermodilution method following the injection of a bolus of a cold saline solution. The measurements are repeated with blood flow being incrementally increased until a noticeable recirculation is ascertained. At this blood flow rate, the fistula flow Q.sub.F is then exceeded. Associated with this method is the problem that the fistula flow is limited only to the measuring interval and, therefore, cannot be precisely determined, and that the measurement is costly because of the continuous bolus injection. It is more significant, however, that this work does not consider that a recirculation, namely the cardiopulmonary recirculation, is present in vivo in every blood flow, and overlaps the fistula recirculation. Aldridge et al. discuss still a second method which seems better suited for detecting fistula flow. When a variation in the extracorporeal blood flow Q.sub.B causes it to closely approach fistula flow Q.sub.F, an oscillation is observed in the arterial temperature TA. This results from the periodically fluctuating pressure conditions in the fistula caused by the action of the blood pump, usually designed as a roller pump. The disadvantage of this method is that the "correct" blood flow Q.sub.B =Q.sub.F can only be found tediously by varying Q.sub.B, and by checking TA for the presence of oscillations. Moreover, the temperature sensory mechanism in the arterial system must exhibit a very low response time, since the oscillation period lies within the range of a few tenths of a second up to about one second. This requirement can only be met, intra-arterially, by placing the sensors directly in the bloodstream, which is not a viable method for routine treatments.